In electrochemical transistors (ECTs), the transistor channel is bridged by an electrochemically active, conducting polymer such as p-doped poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT-PSS). Adjacent to the conducting polymer layer, an electronically insulating but ionically conducting electrolyte layer in contact with a counter electrode (“gate electrode”) allows an electric field to be applied to the electrochemically active material in the transistor channel, thereby changing its oxidation state. When the conducting polymer is reduced from the p-doped (conducting) to its neutral (semiconducting) form, its conductivity drops drastically.
ECTs based on conducting polymers that are stable in their p-doped state operate in a depletion mode. Prior to application of the gate voltage, the transistor is in an electrochemically stabilized ON-state where negatively charged counter-ions are present in the bulk of the conjugated polymer, balancing the positive charges on the polymer chains and thereby allowing for much higher charge carrier densities as compared to conventional organic field effect transistors (OFETs) in their ON-state. Due to this increased charge carrier density, ECTs can deliver much higher current levels than OFETs. Application of a positive gate voltage relative to the source contact switches the transistor channel from its initially p-doped ON-state to its electrochemically reduced OFF-state [see US 2006202289, WO 03047009, WO 02071505 and SE 0100748].
Another variant of ECTs is based on organic semiconductors that are stable in their neutral, non-doped state, i.e. the same organic semiconductor materials as are commonly used in OFETs [see T. Masateru and K. Tomoji, “Vertical electrochemical transistor based on poly(3-hexylthiophene) and cyanoethylpullulan”, Appl. Phys. Lett. 85, 3298 (2004); S. Chao and M. S. Wrighton, “Solid State Microelectrochemistry: Electrical Characteristics of a Solid State Microelectrochemical Transistor Based on Poly(3-methylthiophene)”, J. Am. Chem. Soc. 109, 6627 (1987); and T. G. Backlund, H. G. O. Sandberg, R. Österbacka, and H. Stubb, “Current modulation of a hygroscopic insulator organic field-effect transistor”, Appl. Phys. Lett. 85, 3887 (2004)].
As in ECTs based on conducting polymers, the neutral organic semiconductor in the transistor channel is gated electrochemically with an electrolyte comprising mobile ions. However, these devices work in an accumulative mode, i.e. prior to the application of the gate voltage, the transistor is in its non-doped OFF-state. Applying a negative voltage to the gate (counter) electrode then results in electrochemical doping of the transistor channel so that the transistor is switched to its ON-state.
Both variants of ECTs have the benefit of operating at very small gate voltages. Typically, the gate voltage |VG| required to switch an ECT is around 0-2V. The electrochemical doping and de-doping processes of the material in the transistor channel are driven by the potential applied between the gate (counter) electrode and the polymer in the transistor channel (acting as the working electrode). In contrast to OFETs, where the electric field extends throughout the dielectric layer between the transistor channel and the gate electrode, the electric field in electrolyte-gated ECTs is confined to electrolytic double layer capacitances formed at the interfaces between the electrolyte and the transistor channel and between the electrolyte and the gate electrode. As the specific capacitance of the electrolytic double layers (of the order of οF/cm2) is far larger than the specific capacitance in conventional OFETs (of the order of nF/cm2), the gate voltages required for switching ECTs are much lower than those for OFETs [see M. J. Panzer, C. R. Newman, and C. D. Frisbie, “Low-voltage operation of a pentacene field-effect transistor with a polymer electrolyte gate dielectric”, Appl. Phys. Lett. 86, 103503 (2005)].
A major disadvantage of ECTs is the fact that, in comparison to OFETs, their switching times are very long. This is due to the fact that in order to maintain charge neutrality, the electrochemical doping and de-doping of the transistor channel of an ECT requires both the exchange of counter-ions between the transistor channel and the electrolyte, and the diffusion of ions within the electrolyte, i.e. between the transistor channel and the gate electrode. The switching rate of ECTs therefore depends both on the mobility of the counter-ions within the transistor channel and on the ionic conductance of the electrolyte.
Recently, an electrolyte-gated field-effect transistor based on a p-type semiconducting polymer that is gated via a polyanionic proton conductor has been described [see E. Said et al., “Polymer field-effect transistor gated via a poly(styrenesulfonic acid) thin film”, Appl. Phys. Lett. 89, 143507 (2006)]. Similarly to the ECTs based on semiconducting polymers described above, the application of a very small gate voltage (<1V) to this transistor is sufficient to result in the formation of large electrolytic double layer capacitances at the semiconductor-electrolyte and electrolyte-gate electrode interfaces, and hence the turning-on of the device.
In the above field-effect transistor gated via a polyanionic proton conductor, the electrolyte comprises only immobile polymeric anions that cannot diffuse into the bulk of the semiconductor layer. Thus, the rate limiting electrochemical doping and de-doping of the semiconductor layer that occurs in other ECTs is prevented. The device is therefore not an electrochemical device, but rather a field-effect transistor (FET) gated via an electrolyte. In combination with the high ionic conductivity of the proton conducting electrolyte, the prevention of electrochemical doping and de-doping allows the device to provide response times of the order of milliseconds and corresponding device operation in the kHz frequency range. The device retains the low driving voltages exhibited by ECTs.
Two major problems with electrolyte-gated transistors are their low operation frequencies and the occurrence of hysteresis during switching. The switching speed of an electrolyte-gated transistor depends on the rate of formation of the electrolytic double layer capacitance at the semiconductor-electrolyte interface, which is limited by the ionic conductance of the electrolyte between the transistor channel and the gate electrode. The ionic conductance of the electrolyte depends on the mobilities and the concentrations of the different ionic species present in the electrolyte. The ionic conductance also depends on the distance between the transistor channel and the gate electrode (i.e. the device geometry).
In the case of ECTs, an additional factor limiting the switching speed is the rate of diffusion of counter-ions into the bulk of the semiconductor material in the transistor channel.
A contributing factor to the occurrence of hysteresis during switching of both electrolyte-gated FETs and ECTs is the transient inhomogeneity of the electric field along the width of the transistor channel when a laterally positioned gate electrode is used.
Another problem with electrolyte-gated transistors is the leakage and diffusion of ions from the electrolyte into other device components. The performance of electric field-driven devices such as electrophoretic displays (EPDs) deteriorates in the presence of mobile ions, which results in ionic leakage currents and hysteresis effects. Therefore, great care has to be taken to encapsulate the electrolyte when electrolyte-gated transistors are used in a system including electric field-driven devices.
An additional problem with proton-conducting electrolytes, such as that used in the electrolyte-gated FET discussed above, is that they are highly acidic and therefore corrosive. The problem is exacerbated in liquid electrolytes, where leakage of the electrolyte may occur.
Finally, the stability of the electrolyte itself is a major problem for electrolyte-gated transistors in general. Both in ECTs and in electrolyte-gated FETs, the ionic conductivity of the electrolyte needs to be maintained at a constant level.
This requirement is difficult to fulfill because both water-based and solvent-based electrolytes are volatile. Due to their volatility, the electrolytes eventually dry out and hence their ionic conductivity decreases. Furthermore, in the case of non-aqueous electrolytes (e.g. solid polymeric electrolytes such as Li-salts dissolved in polyethylene oxide (PEO)) the ionic conductivity is often strongly dependent on trace amounts of residual water present in the electrolyte. Electrolytes are inherently polar and therefore tend to absorb water from the environment, i.e. electrolytes are hygroscopic.
Therefore, in addition to the loss of water or volatile organic solvents from the electrolyte, absorption of water from the environment by the electrolyte must be prevented in order to maintain a constant ionic conductivity.